Indirect Methods of Dating Seafloor Activity: Geology, Regional Stratigraphic Markers, and Seafloor Current Processes

Abstract

The complex geologic development of the Sigsbee Escarpment in the Southern Green Canyon area has been studied in great det ail using a comprehensive suite of geophysical and geotechnical data. Integration of these data led to the recognition of certain geologic indicators that assist, either directly, or indirectly, in "filling the gaps" in the geologic history of the Atlantis area. These indicators include:the Law of Superposition,regional stratigraphic markers,sedimentation rates,seafloor current processes, andfault offsets. These indicators provide both relative and absolute timing constraints on the occurrence and level of activity of various geologic depositional and erosional processes in the Atlantis prospect area. They are most beneficial where quantitative dating methods (i.e. radiocarbon dating) cannot be used or do not adequately constrain the timing and relationship of geologic processes and events. In this way, they play a vital role in assessing the past, present, and potential future rates geologic activity that is critical to the success of the Atlantis development. Introduction BP America, Inc., and its partners are currently evaluating the feasibility of installing various production facilities along the Sigsbee Escarpment in the southern Green Canyon area, Gulf of Mexico (Figure 1). Current development plans call for the seabed installation of architecture such as anchors, manifolds, flowlines, umbilicals, and export pipelines. The complex geologic development of the Sigsbee Escarpment has been studied in great detail using an integrated suite of geophysical and geotechnical data. These data were correlated to portray the chronology of active geologic processes over roughly the past 30,000 years. Integration of these data also led to the recognition of certain geologic indicators that assist, either directly, or indirectly, in "filling in the gaps" in the geologic history of Atlantis. These indicators include:the Superposition of stratigraphy and geologic features,the Triplet and other regional stratigraphic markers,sedimentation rates,seafloor current processes, andfault offsets. Use of these indicators provided a relative sequence for the occurrence and level of activity of various geologic processes in the Atlantis area where the quantitative dating methods (i.e. radiocarbon or paleontological) cannot be used or do not reveal significant insights into the timing and relationship of geologic processes and events. Thus, using indirect geologic indicators of sequencing and dating of geologic processes at Atlantis have been key in understanding the geologic history and rates of activity of geologic processes. When integrated with the entire suite of data, they contribute vital information about past, present, and future rates of activity for geologic processes; an understanding of which is critical to the success of the Atlantis field over its projected lifetime Available Data Several data types were used to identify and correlate the aforementioned geologic indicators including geophysical, geotechnical, radiocarbon dating, and various laboratory tests. The geophysical data consisted of two 3D speculative surveys, 2D high-res seismic lines, two 3D high-res seismic surveys, and shallow high-resolution data consisting of multibeam sounding, side-scan sonar, and subbottom profiler. The geotechnical field study programs consisted of 4- in. jumbo piston cores, 3-in conventional piston cores, box cores, and deep soil borings.

Age Dating of Past Slope Failures of the Sigsbee Escarpment within Atlantis and Mad Dog Developments

“…The dynamic behavior of the Sigsbee Salt Nappe as described in the companion papers (1,2) is the single most important influence in the geologic and geomorphic development within the shallow section (upper 3,000 ft) in the area of the Atlantis and Mad Dog Developments. The Sigsbee Escarpment represents the topographic expression of a thin-skinned sediment section overlying a series of enechelon, coalescing, Pleistocene emplaced allochthonous salt nappes (3, 4, 5, and 6).…”

Sedimentation in the Vicinity of the Sigsbee Escarpment during the Last 25,000 yrs

“…The face of the escarpment is marked by a series of slump scars and the debris from these slumps and other mass wasting processes is deposited on top of preexisting hemipelagic drape or mass wasting debris at the base of the slope and on the rise (3,4,6). The face of the escarpment is marked by a series of slump scars and the debris from these slumps and other mass wasting processes is deposited on top of preexisting hemipelagic drape or mass wasting debris at the base of the slope and on the rise (3,4,6).…”

“…The face of the escarpment is marked by a series of slump scars and the debris from these slumps and other mass wasting processes is deposited on top of preexisting hemipelagic drape or mass wasting debris at the base of the slope and on the rise (3,4,6). Also, sediments may be eroded and redeposited by bottom water currents (3,5). The sediments may be displaced by several styles of faults (1).…”

“…The purpose of the coring was to characterize the various types of sediment deposits, morphologic and structural features, and to ground-truth seafloor backscatter amplitudes and subbottom seismic reflectors (1,3,4,5,6,9). A total of 95 cores were collected and their lengths ranged from several to nearly 20 m. In addition, 8 borings were also collected to investigate deeper sediments.…”

Integrated Studies To Characterize the Mad Dog Spar Anchor Locations and Plan Their Installation

“…View of the "triplet" regional marker in piston cores, impedance profile, and sub-bottom profiler records (modified fromBrand et al 2003) View of the "triplet" regional marker in piston cores, impedance profile, and sub-bottom profiler records (modified fromBrand et al 2003) …”